1. Technical Field
The present invention relates to a fluorescent light emitting element and a projector including the fluorescent light emitting element.
2. Related Art
In the related art, a projector that modulates light emitted from a light source according to image information and projects the light is known. In the related art, a discharge type light source is used as the light source for such a projector. However, in recent years, a light emitting element such as a light emitting diode (LED) or a semiconductor laser is used in many cases. Further, a fluorescent light emitting element that converts a wavelength of light such as semiconductor laser using a phosphor is also used.
JP-A-2011-53320 discloses a light source device that includes an excitation light source and a phosphor layer which emits fluorescence using excitation light emitted from the excitation light source, in which the phosphor layer is formed of phosphors and a binder in which the phosphors are uniformly inlaid.
However, in the light source device disclosed in JP-A-2011-53320, since the binder as a base material is a resin such as silicone, the heat resistance of the phosphor layer becomes large. For this reason, there is a problem in that the temperature of the phosphor is increased in response to an increase in the light amount of the excitation light which is incident on the phosphor layer and the increased temperature causes temperature quenching of the phosphor. Further, in a case where the thickness of the phosphor layer is made extremely thin for a purpose of lowering the heat resistance of the phosphor layer, there is a problem in that excitation light absorptivity (absorbed amount of excitation light) of the phosphor layer is insufficient, fluorescence conversion efficiency of the excitation light is not improved, and thus, an improvement in output of the light source device may not be expected.
In addition, there is a correlation between concentration of a fluorescent substance (in a case of a YAG:Ce phosphor, Ce3+ ion) in the inside of the phosphor and fluorescence quantum efficiency (the number of photons generated as fluorescence/the number of photons of excitation light absorbed in fluorescent substance) of the phosphor, and a phenomenon (concentration quenching) in which fluorescence quantum efficiency is decreased as the concentration of a fluorescent substance is increased is known in general.
In a phosphor layer obtained by mixing a binder and a phosphor in the related art, in a case where the thickness of the phosphor layer is made thin for a purpose of lowering the heat resistance, it is necessary to increase the concentration of a fluorescent substance so that excitation light absorptivity is not decreased. However, as described above, there is a problem in that the fluorescence quantum efficiency is decreased when the concentration of the fluorescent substance is increased and sufficient light emitting efficiency may not be obtained.
As a method of resolving the above-described problems, a method of arranging a sintered body, which is formed of only a phosphor without a binder, on a reflection mirror as it is exemplified. According to the method, heat resistance of a phosphor layer can be reduced using high thermal conductivity of the phosphor. Further, since the total amount of a necessary fluorescent substance can be secured even when the concentration of the fluorescent substance is not increased, necessary excitation light absorptivity can be secured. However, an optimal combination of the concentration of a fluorescent substance and the thickness of a phosphor layer, which can obtain sufficiently high light emitting efficiency, has not been disclosed.
Accordingly, a fluorescent light emitting element capable of obtaining high output by maximizing light emitting efficiency using an optimal combination of the concentration of a fluorescent substance and the thickness of a phosphor layer; and a projector including such a fluorescent light emitting element have been demanded.